In wavelength division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is generally known that optical fibers offer much higher bandwidth than conventional coaxial communications; furthermore a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). By transmitting several channels at different optical wavelengths into an fiber (i.e., wavelength division multiplexing, or WDM), this bandwidth may be more efficiently utilized.
There have been many attempts to develop a compact, high resolution waveguide demultiplexer or spectrometer for application in areas such as spectroscopy, optical networks and optical links and more particularly optical communication systems. Such a demultiplexer can be extremely critical in wavelength division multiplexing (WDM) links. In these links or networks, each channel is assigned a distinct and unique wavelength for data transmission.. Thus, the optical fiber that connects channels in a WDM network carries many discrete wavelength channels and a particular wavelength is selected before the data is received. The data reception can be achieved by combining a wavelength demultiplexer, photodetectors and electronic selection circuitries. In WDM links, many wavelengths are multiplexed and transmitted through a single optical fiber to increase the capacity of the fiber. The receiver must demultiplex the many wavelengths and select the proper channel for reception. In these applications, the requirements on the wavelength demultiplexer are typically: an optical bandwidth &gt;30 nm, a wavelength resolution of a few angstroms, polarization insensitivity, compactness, low loss, low crosstalk, and a low manufacturing cost.
At present, there are many known methods of selecting particular wavelengths, however, none are ideal for the applications outlined above. Such methods rely either on bulk optics or waveguide structures where the frequency selective element is either an interference grating or a Fabry-Perot (F-P) cavity. Bulk optics are generally too large and expensive for fiber based WDM applications. Diffraction gratings have been known for many years and produce a high resolution spectrum where the wavelength is a function of the diffracted angle. Thus a single grating can demultiplex many wavelengths. However, available bulk gratings have generally been expensive and difficult to use with optical fibers; another known drawback to these grating is their large physical size.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising the multiplexed channel and plural optical fibers comprising the plural demultiplexed channels are described in various U.S. patents. For example, multiplexing/demultiplexing with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745,991. Multiplexiing/demultiplexing using optical bandpass filters (such as a resonant cavity) is disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478. Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat. No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 disclose optical coupling between adjacent waveguides to achieve a demultiplexing function. A similar technique is disclosed in U.S. Pat. No. 4,900,118. Unfortunately, the foregoing techniques are limited by their discrete components to a small number of wavelengths in the multiplexed channel.
One way of overcoming such a limitation is to employ diffraction gratings to perform the multiplexing and demultiplexing functions as is shown by U.S. Pat. Nos. 4,111,524 and 4,993,796. Close spacing of the multiplexed and demultiplexed channels makes fabrication awkward and increases the likelihood of cross-talk. One way of overcoming this latter difficulty is to employ a curved diffraction grating which reflects the incoming signal at right angles. These curved gratings are known as Echelle gratings. Various such gratings are described in U.S. Pat. No. 5,206,920 in the name of Cremer et at; Appl Phys Lett Vol. 58 No. 18 May 1991 p1949 Soole et at; Phot. Tech. Letts. Vol. 4 No.1 1992 p 108 Cremer et at.; Appl Phys Lett Vol. 61 No.23 Dec. 1992 p 2750 Soole et al.; Elec. Letts. March 1994 vol. 30 No. 6 p 512 Poguntke et at.; Elec. Letts. Sept. 1994 Vol. 30 no. 19 p 1625 Cremer et at; and, Phot. Tech. Letts. Vol. 6 no. 9 p 1109 Clemens et al. One of the difficulties associated with the manufacture of Echelle gratings is that they require the etching of high quality facets with well controlled side-wall angle and low rounding. Manufacturing and fabrication precision problems associated with devices of this type are outlined in Journal of Lightwave Tech. Vol. 12 No.11 Nov. 1994 p1939 Wu and Chen; Elec. Letts. April 1994 Vol. 30 No. 8 p 664 Soole et al. This results in higher actual on chip losses (.about.4 dB) than predicted (.about.1 dB) on the basis of fabrication tolerances. Furthermore, the lithography of such gratings using focused ion beam or electron beam lithography is very time consuming. If UV lithography is used, the grating must be operated at very high orders.
The foregoing limitation is overcome in a technique in which a diffraction grating is combined with a lens, as disclosed in U.S. Pat. Nos. 4,777,663 and 4,839,884 and 4,367,040 and 4,739,501. The advantage of the lens and grating combination is that the plural optical fibers of the demultiplexed channels may interface directly with the single optical fiber of the multiplexed channel through the grating and lens combination. The problem with the lens and grating combination is that the lens is a large discrete component. Moreover, the diffraction grating itself is typically a discrete component. A related technique is disclosed in U.S. Pat. No. 5,107,359 employing two discrete components, namely either two diffraction gratings or a grating and a specular reflection surface.
Accordingly, the foregoing techniques suffer from drawbacks for integrated circuit implementation and are therefore relatively large expensive devices incapable of exploiting the advantages of integrated circuits in WDM discussed above on a manufacturable scale.
Thus, there is a need for compact, manufacturable wavelength division multiplexing(WDM) device for telecommunications purposes and for other applications such as compact spectrometers which also require similar design.
It is therefore an object of this invention to provide a transmissive refraction grating that is relatively inexpensive to manufacture and compact.
It is a further object of this invention to provide a transmissive refraction grating that operates in the Raman-Nath regime.